There is well known a technique for shifting a transmission having a plurality of frictional coupling devices, with a clutch-to-clutch shifting operation (a releasing/engaging switching operation) in which one of the frictional coupling devices is brought into a released state while another of the frictional coupling devices is brought into an engaged state. In this clutch-to-clutch shifting operation, the shifting action of the transmission is controlled according to changes of torque capacities of the two relevant frictional coupling devices. To implement the shifting action, therefore, the torques to be transmitted through these two frictional coupling devices are required to be intricately controlled.
When a shift-up action of the automatic transmission is performed with the clutch-to-clutch shifting operation, for instance, the torque capacity of the frictional coupling device to be engaged is gradually increased in a torque phase of the shift-up action, and a rate of increase of the torque capacity of that frictional coupling device is held relatively low after the shift-up action has entered an inertia phase, so that an operating speed of an engine (namely, a rotating speed of an input shaft of the automatic transmission) is controlled while a rate of increase of an output torque of the automatic transmission in the inertia phase is held relatively low.
That is, the frictional coupling device to be engaged has effect on both of the rate of change of the torque capacity of the relevant frictional coupling device in the torque phase, and the change of the operating speed of the engine in the inertia phase. Accordingly, where the rate of increase of the torque capacity of the relevant frictional coupling element is increased to reduce a period of time of the torque phase, there is a risk of delay or variation in the control to reduce the rate of increase of the torque capacity in the inertia phase. In this case, an excessively large value of the torque capacity of the relevant frictional coupling element in the inertia phase may cause generation of a shifting shock of the automatic transmission.
In view of the problem described above, there has been proposed to shift the automatic transmission by controlling three frictional coupling devices consisting of the two frictional coupling devices associated with the clutch-to-clutch shifting operation and an input clutch through which a vehicle drive force is transmitted to the automatic transmission. Patent Document 1 discloses an example of this proposal. Namely, Patent Document 1 discloses a technique to reduce a torque capacity of the input clutch for limiting the torque to be transmitted to an automatic transmission during the shifting action.